The present invention relates to apparatus for measuring the oscillation frequency of a voltage controlled oscillator for use as a first local oscillator of a spectrum analyzer, in particular, a voltage controlled oscillator whose oscillation frequency is as high as several gigahertz.
In a spectrum analyzer for measuring an input signal of a high frequency, the frequency of a first local signal which is employed for converting the input signal frequency to a first intermediate frequency (e.g., a 4 GHz signal) is varied, for example, in the range of 50 to 200 GHz. As a voltage controlled oscillator which oscillates at such a high frequency and whose oscillation frequency can be controlled by a voltage, use is made of, for instance, a YIG oscillator. To measure the input signal frequency in the spectrum analyzer, it is necessary to measure the frequency of its first local signal. In order to measure the oscillation frequency of an oscillator of such a high frequency, however, a special frequency measuring apparatus using many ultra-high-frequency elements is needed, and because of this, the measurement becomes markedly expensive. In view of this, it is the common practice in the prior art to measure a control voltage of the voltage controlled oscillator and obtain the corresponding oscillation frequency from the measured result. With such a method, however, a frequency of several gigahertz can be measured only with an accuracy of 1 to 1 MHz, that is, with an accuracy of only three digits.